Page 330 - Radar Technology Encyclopedia

PULSER RADAR 320

mod-anode) pulser, and grid pulser. The first is typically an with a given doppler shift, which differs from the doppler
active switch that is either driven hard enough to bring its shift of the target by the value f . IAM
d
voltage drop as low as possible, minimizing dissipation and Ref.: Skolnik (1970), p. 3.40.
maximizing efficiency, or operating as a constant-current
QUANTIZATION. Quantization is “a process in which the
device by limiting its drive. The basic circuit implementations
range of values of the wave is divided into a finite number of
are direct-coupled, capacitor coupled, transformer-coupled,
smaller subranges, each of which is represented by an
or capacitor-and-transformer-coupled. A basic type of mod-
assigned (or quantized) value within the subrange.” In radar
anode pulser is floating-deck modulator used, for example,
applications it is performed in the process of analog-to-digital
with a klystron. It can provide very good pulse flatness due to
conversion, consisting of two functions: waveform sampling
the usage of capacitor bank and sufficient pulse lengths as
(discretization in time), and quantization (discretization in
there is no limit on maximum pulsewidth except for capacitor
amplitude). (See also CONVERTER, analog-to-digital;
bank size. The grid pulser is the smallest, easiest, and least
ERROR, quantization.) SAL
expensive type of pulser, but it can be used only in RF tubes
Ref.: IEEE (1993), p. 1,046; Barton (1969), p. 187.
with the grids (see also MODULATOR). SAL
Ref.: Skolnik (1990), p. 4.37.
R
Q
“(RUNNING) RABBITS” are asynchronous pulses interfer-
ing with target visibility on a display. When the interfering
Q(-FACTOR). The Q factor characterizes the resonant prop-
pulse train differs from the radar pulse repetition interval by
erties of an oscillatory system. Series and parallel tuned cir-
Dt, the rabbits move in range by DR = cDt/2 per PRI, corre-
cuits are widely used in radars as oscillatory systems. For
sponding to a velocity cDt/2t . Thus, two radars having the
r
them, resonant frequency:
same nominal PRI but differing by a timing oscillator offset
1 - 5
w = ----------- of 0.001% will create rabbits moving at 10 c/2 = 1500 m/s,
r
LC potentially interpreted by an untrained operator as high-speed
where L is the inductance and C is the capacitance of the cir- targets. Even if the rabbits are recognized as interference,
cuit. their presence may detract from target detection. DKB
For a series tuned circuit, the Q-factor is Ref.: Johnston (1979), p. 65.
U Lr U Cr X
Q = --------- = --------- = --- RADAR is “a device for transmitting electromagnetic signals
1
U
i U i R and receiving echoes from objects of interest (targets) within
where U Lr and U Cr are the voltages across the inductor and its volume of coverage. Presence of a target is revealed by
capacitor, U is the input voltage, X = w L = 1/w C is the detection of its echo or its transponder reply. Additional infor-
i
r
r
impedance at the resonant frequency, and R is the resistance mation about a target provided by a radar includes one or
of the circuit. more of the following: distance (range), by the elapsed time
For a parallel circuit: between transmission of the signal and reception of the return
I k signal; direction, by use of directive antenna patterns; rate of
Q = ----
2 I change of range, by measurement of doppler shift; description
where I is the current in the circuit and I is the current in a or classification of target, by analysis of echoes and their vari-
k
common conductor at circuit input. ation with time. The term radar was originally an acronym
Thus, for a series circuit, Q-factor illustrates the extent to for radio detection and ranging. Some radars can also operate
which voltage at the output is greater than that at circuit input. in a passive mode in which the transmitter is turned off and
For a parallel circuit, Q-factor illustrates the extent to which information about targets is derived by receiving radiation
current at circuit output is greater than that at the input. AIL emanating from the targets themselves or reflected by targets
from external sources.”
Ref.:Terman (1955), p. 45; Gonorovskiy (1986), p. 120.
Radar is also recognized as the field of science and tech-
The Q-FUNCTION describes the dependence of the cross-
nology that includes the methods and equipment to perform
sectional area of the square of the modulus of the ambiguity
the following basic operations against the targets of interest:
)
function c(×, drawn parallel to the axis of the time delays, on
(1) Radar detection (see DETECTION).
the doppler frequency shift f : (2) Radar measurement (see MEASUREMENT).
d
¥ (3) Radar recognition, discrimination, and identification
t
2 j2pf d
Q f () = ò c t 0 )e d t (see TARGET RECOGNITION AND IDENTIFI-
(
,
u
d
u
CATION ).
– ¥
It is used in determining the ability of a signal to suppress The quantitative and qualitative description of radar operation
interference from local objects uniformly distributed in range provides radar performance figures, the most basic of which